HARRISBURG, Pa. (AP) -- A state appeals court has upheld the life-without-parole sentence imposed on a soldier in his 20s who was convicted of killing his then-14-year-old girlfriend's mother because she disapproved of their relationship.

Barnes was 20 when Silvonek was killed and is now 23. The Fort Meade, Maryland, soldier argued his statements to police should have been suppressed because he was under duress, but the court said police never tried to intimidate him and he never sought a lawyer or an end to the interrogation.

The panel also found no fault with testimony from Barnes' girlfriend, Jamie Silvonek, and a friend of hers who reported overhearing her and Barnes talking about killing her parents.

Barnes stabbed the victim in her car after Barnes, Cheryl Silvonek and Jamie Silvonek returned home from a concert, authorities said. The woman had threatened to report Barnes to police for having sex with her underage daughter, prosecutor said.

Jamie Silvonek pleaded guilty this year and is serving 35 years to life. She testified at Barnes' trial that she plotted the murder and urged her boyfriend, in a series of texts, to carry it out.

Jurors convicted Barnes of first-degree murder, rejecting his argument at trial that the girl killed her mother and he only helped dispose of the body later because she said she was pregnant.